(1) Field of the Invention
The present invention relates generally to abrasives and abrasive tools suitable for surface grinding and polishing of hard and/or brittle materials. This invention more particularly relates to porous, bonded abrasive articles having an interconnected pore structure and methods for making same.
(2) Background Information
The use of porous abrasive products to improve mechanical grinding processes is well known. Many high performance abrasive products require a controlled amount of porosity, and interconnected porosity, to be engineered in to the structure of vitrified and resin bonded grinding wheels. Pores typically provide access to grinding fluids, such as coolants and lubricants, which promote efficient cutting, minimize metallurgical damage (e.g., surface burn), and maximize tool life. Pores also permit the clearance of material (e.g., chips or swarf) removed from an object being ground, which is important especially when the object being ground is relatively soft or when surface finish requirements are demanding.
Previous techniques used to fabricate vitrified bonded abrasive articles and/or tools having porosity have involved creating a pore structure by the addition of organic pore inducing media into the abrasive article. A number of pore inducing materials have been used historically. Examples include PDB/Naphthalene, walnut shells, com, peach pits, and carbon balls. These media sublime or thermally decompose upon firing, leaving voids or pores in the cured abrasive tool. Examples of abrasive tools fabricated in this manner are disclosed in U.S. Pat. No. 5,221,294 to Carmen, et al., and U.S. Pat. No. 5,429,648 to Wu, and Japan Patents A-91-161273 to Grotoh, et al., A-91-281174 to Satoh, et al.
There are, however, drawbacks associated with using these pore inducers. Relatively long, slow burnout cycles are generally required to volatilize and remove the pore inducers from vitrified bonded wheels. Moreover, the volatilized materials are often environmentally hazardous. This off-gassing procedure also tends to be a significant cause of rejections in manufacturing, as it often leads to structural changes and cracking in the fired wheels.
Other examples of porous abrasive tools are disclosed in U.S. patent application Ser. No. 09/990,647 (the ""647 application), entitled Porous Abrasive Tool and Method For Making the Same, filed Nov. 21, 2001, which is fully incorporated herein by reference.
As market demand has grown for precision components in products such as engines, bearings, and electronic devices (e.g., silicon and silicon carbide wafers, magnetic heads, and display windows) the need has grown for abrasive tools for fine precision grinding of a range of relatively hard and/or brittle materials and soft, heat-sensitive materials. Similarly, the need has grown for environmentally friendly techniques for fabricating such tools. Therefore, there exists a need for an improved method of fabricating porous abrasive articles and tools, and for the abrasive articles and abrasive tools produced thereby.
One aspect of the present invention includes a method for fabricating an abrasive article having pores. The method includes blending a mixture of abrasive grain, bond material, and pore inducer, and pressing said mixture into an abrasive laden composite. The composite is exposed to a supercritical fluid for a period of time suitable to dissolve at least a portion of the pore inducer. The pore inducer is soluble in the supercritical fluid, and the abrasive grain and bond material are substantially insoluble in the supercritical fluid. The composite is then thermally processed.
Another aspect of the present invention includes a method for fabricating an abrasive article having from about 40 to about 85 volume percent porosity. The method includes blending a mixture of abrasive grain, non-metallic bond material, and pore inducer, said mixture including from about 30 to about 48 volume percent abrasive grain, from about 4 to about 20 volume percent bond material, and from about 1 to about 36 volume percent pore inducers. The mixture is pressed into an abrasive laden composite, and exposed to a supercritical fluid for a period of time suitable to dissolve at least a portion of said pore inducer. The abrasive grain and bond material are substantially insoluble in the supercritical fluid. The composite is then thermally processed.